1. Technical Field of the Invention
The present invention relates in general to the mobile communications field and, in particular, to a method for controlling transmission power levels in a spread spectrum or Code Division Multiple Access (CDMA) cellular communication system.
2. Description of Related Art
In wideband spread spectrum cellular communication systems (often referred to as Wideband-CDMA or WCDMA systems), power control symbols are transmitted on a regular basis in order to compensate for the effects of changing conditions that influence the channel, such as fading and shadowing. For example, a base station can transmit a power control command which signals a served mobile station to increase or decrease its transmission power level. The mobile station can then decide whether to increase or decrease its transmission power level in response to the power control command received.
As such, it is important that the power control algorithms used in CDMA systems be designed to maintain the negotiated quality of the data channels for all active users. Essentially, the basic power control algorithms used in existing systems are designed to implement this capability in each connection, with two nested control loops. The outer (slower) power control loop controls a transmit signal-to-interference ratio (SIR) or signal-to-noise ratio (SNR) target value for use in the inner (faster) closed power control loop so that the actual Quality of Service (QoS) is close to the negotiated QoS. The inner power control loop estimates the SIR of the forward transmission channel, compares the estimated SIR to the SIR target value, and based on the results of the comparison, transmits power control commands on the reverse transmission channel which xe2x80x9cadvisexe2x80x9d the transmitter on the forward channel about whether to increase or decrease its transmission power level. Typically, the same power control principle can be applied for both the uplink and downlink transmissions.
The conventional inner power control loops are designed to function based on SIR measurements that can be obtained (typically for WCDMA systems) at a rate of 1600 per second. The conventional outer power control loops are designed to function based on QoS values that can be obtained at a typical rate of one per second. Typically, is these QoS values are expressed in terms of a frame error rate (FER), bit error rate (BER), packet delay, or speech quality index (SQI). As such, these QoS values can be obtained only at the above-described rate of once per second, or once per code block (typically, 50 to 100 times per second). However, these QoS values should be accumulated over a significant period of time in order to produce statistically reliable results.
The existing outer power control loops can change a SIR target value faster when an appropriate change is known due to a change of service (e.g., when the transmission data rate is changed). In this regard, FIGS. 1A and 1B are related diagrams that illustrate how an existing outer power control loop can respond (by varying the SIR target value) to a change in the channel environment or to a known service change, respectively. As such, for a properly designed power control implementation, the outer power control loop should be unable to vary the SIR target value any faster than the inner power control loop can respond and change the actual SIR. This approach ensures that the actual SIR follows the SIR target value quite closely all of the time.
A significant problem occurs when the above-described power control approaches are being used. For a number of different reasons, the inner power control loop may be unable to achieve the SIR target value as quickly as the outer power control loop can change the SIR target value. This problem can be caused by limitations on the control loop""s bandwidth or on the transmitter""s output power range. A reason for this problem is that the outer power control loop in use may be modified to utilize parameters other than QoS values to estimate the QoS and follow the negotiated QoS at a higher rate. For example, the parameters used may be the mean and standard deviation of the raw BER for a number of code blocks, or a characterization of the type of transmission channel involved. Also, the level of interference in the transmission channel can be due to discontinuous data or abrupt variations in speech transmissions. As such, the combined effects of an outer control loop that is faster than the inner loop, along with a rapidly varying transmission channel, can cause a significant difference between the SIR target value and the SIR actually achieved in the channel.
Another reason that an inner power control loop may be unable to achieve a SIR target value is related to the interaction between the power control function and other system management functions (e.g., admission control and congestion control). For example, on the downlink, a base station transmitter might be required to limit its output power level because of an overload or congestion situation. Similarly, the system manager may desire to change the total amount of data being transmitted, as a result of granting access to a new user or changing user data rates.
As a consequence of the above-described problems, the existing inner power control loops are unable to maintain the actual SIR at the desired SIR target value, and the outer power control loop will try to compensate for that inability by further increasing the SIR target value. However, when these circumstances are different, either because the inner loop has achieved an actual SIR that satisfies the negotiated QoS, or the system is successful in reducing the load, the SIR target will already be changed beyond the appropriate value. Under these circumstances, the inner power control loop drives the power level on the forward channel toward this xe2x80x9cfalsexe2x80x9d target value until it is properly adjusted to by the outer power control loop. As such, this over-control condition can result in oscillations around the SIR and an unnecessarily high system load.
FIG. 2 is a diagram that illustrates such over-control oscillations and a high system load where the SIR target is being controlled by QoS parameters only. As shown in FIG. 2, the SIR target value is changed when the QoS is out of range, and the negotiated QoS is not achievable when the transmitted power is limited. Nevertheless, as described in detail below, the present invention successfully resolves these and other related problems.
In accordance with a preferred embodiment of the present invention, a transmission power control method and system are provided for use in a spread spectrum or CDMA mobile communications system whereby an estimate of the achieved SIR is used as an additional parameter for the outer power control loop function. When the QoS and achieved SIR are both too low, the QoS is ignored and the SIR target value is instead controlled by the achieved SIR. Similarly, when both the QoS and achieved SIR are too high, the QoS is ignored and the SIR target value is instead controlled by the achieved SIR.
An important technical advantage of the present invention is that it is possible to increase the adaptation rate of an outer power control loop without the risk of unbounded oscillations in the SIR target due to transmitter power limitations.
Another important technical advantage of the present invention is that the power control method and system are fully contained at the nodes of the communication network and no additional signalling is needed.
Still another important technical advantage of the present invention is that the power control method and system ensure that each mobile station will request a slow increase of the base station""s transmit power after a congestion situation, which prevents a rapid increase of the total transmitted power, and therefore, can prevent yet another congestion situation from occurring.
As such, one objective of the present invention is to provide a power control scheme which completely suppresses the above-described drawbacks of an inner power control loop being unable to achieve the SIR as quickly as the outer power control loop can change the SIR target value.